1. Field of the Invention
The present invention relates to a transmission for a heavy construction equipment, and more particularly to a transmission for a heavy construction equipment such as a wheel type excavator adopting the hydrostatic drive system as means for transmitting travelling power.
2. Description of the Prior Art
A typical transmission for a heavy construction equipment such as a wheel type excavator is illustrated in FIG. 1. As shown in the drawing, the transmission includes a gear box 111 disposed in a lower travelling body of the heavy construction equipment and a controller 105 disposed in an upper turning body of the heavy construction equipment. The upper turning body is coupled to the lower travelling body by means of a turning joint 112 so that it can turn through a desired angle with respect to the lower travelling body.
The controller 105 includes a pilot pump 101 adapted to supply oil for a gear shifting for a speed change and a lubrication, a pressure control valve 102 communicated with the pilot pump 101 and adapted to control pressure of the oil, and a solenoid value 104 communicated with the pressure control valve 102 and adapted to switch oil path communication between the pilot pump 101 and the gear box 111 in accordance with an electrically switching manipulation of a gear shifting lever 103.
The gear box 111 includes a pair of different-stage cylinders 106 and 107 actuated by the oil path communication obtained by the solenoid valve 104, a gear shifting valve 108 provided with oil paths respectively communicated with the cylinders 106 and 107 and adapted to perform a switching operation for selectively opening and closing the oil paths, and a pair of different-stage clutches 109 and 110 both communicated with the gear shifting valve 108 and actuated by the oil path communication switching operation of the gear shifting valve 108.
Although not shown, the turning joint 112 has various oil paths respectively adapted to supply oil pressure from a hydraulic pump equipped in the lower travelling body to various working units such as a boom of the upper turning body or a dipper stick.
Since various oil paths pass through the turning joint 112 as a moving part in the above-mentioned conventional transmission, damage of the turning joint 112 which occurs relatively frequently results in oil leakage from oil paths and thereby loss of pressurized oil.
In the conventional transmission, the oil paths extending from the hydraulic pump equipped in the gear box 111 as an oil pressure supply source respectively to the cylinders 106 and 107 are relatively long. Due to such a long oil path length, a considerably high frictional resistance occurs in the oil paths, thereby resulting in a degradation in pressure stability characteristic of the clutches 109 and 110.
When the pressure of oil applied to the clutch 109 or the clutch 110 is decreased due to the loss of pressurized oil or the degraded pressure stability characteristic, adjacent discs of the clutch in question rotate while being in insufficient contact with each other. As a result, a slippage occurs between the discs being in contact with each other, thereby causing severe frictional heat to be generated. Such a severe frictional heat results in damage and early worn-out of the clutch.
Where the oil paths passing through the turning joint 112 are damaged due to the damage of the turning joint 112, the pressurized oil supplied to the cylinders 106 and 107 for the gear shifting may be mixed with the oil for lubrication or the pressurized oil for actuating the working unit. In this case, the overall hydraulic system may be adversely affected to a considerable extent.